Acute flaccid myelitis (AFM) or Acute flaccid paralysis is classically defined as an acquired spinal cord disorder that presents with the rapid onset of weakness in one or more limbs due to any inflammatory or infectious process that affects the anterior horn cells. This is typically secondary to direct infection or post-infectious inflammation from enteroviruses including Enterovirus D68 and Enterovirus A71.
Diagnostic criteria:
The current 2020 CDC definition for confirmed AFM includes a patient with rapid onset of flaccid weakness in one or more limbs, with an MRI showing a spinal cord lesion predominantly involving grey matter and spanning one or more vertebral segments. This definition excludes individuals with other causes of grey matter spinal lesion, such as diagnosed malignancy, vascular disease or anatomic abnormalities. Note, there is no requirement for isolating a culprit virus in order to make the diagnosis.
A presumptive diagnosis may be made if the white matter lesions involve grey matter, but the predominance cannot be determined. This is a reportable illness, due to recent outbreaks ongoing in North America since 2014. Rare with an incidence of < 1/1,000,000, mainly occurring in children.
On imaging
MRI of the spinal cord typically shows longitudinally-extensive T2-hyperintense lesions restricted to or predominantly involving the spinal grey matter. Acutely, these lesions may be ill-defined and extending the length of the central spinal cord grey matter. Subacute imaging reveals more circumscribed lesions with a predilection for the anterior horn.There may or may not also be enhancement of the ventral spinal nerve roots or anterior horn cells.
MRI brain may show a nonenhancing T2 hyperintensity of the dorsal pons and medulla and/or substantia nigra, as these enterovirus have also been known to cause a polio-like rhombencephalitis.. There may also be enhancement of cranial nerves.
References:
Author: Dr. Risa Namsechi